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Indole-3-Carbinol

Summary

Indole-3-carbinol (I3C) is derived from the hydrolysis (breakdown)
of glucobrassicin, a compound found in cruciferous vegetables.
(More Information)

In the acidic environment of the stomach, I3C molecules can
combine with each other to form a number of biologically active
acid condensation products, such as 3,3'-diindolylmethane (DIM).
(More Information)

I3C has been found to inhibit the development of cancer in animals
when given before or at the same time as a carcinogen.
However, in some cases, I3C enhanced the development of cancer
in animals when administered after a carcinogen. (More
Information)

The contradictory results of animal studies have led some experts
to caution against the widespread use of I3C and DIM supplements
for cancer prevention in humans until their potential risks and
benefits are better understood. (More Information)

Although I3C and DIM supplementation have been found to alter
urinary estrogen metabolite
profiles in women, the effects of I3C and DIM on breast cancer
risk are not known. (More Information)

Cruciferous vegetables differ from other classes of vegetables
in that they are rich sources of sulfur-containing compounds known as
glucosinolates (see Cruciferous
Vegetables). Because epidemiological
studies provide some evidence that diets rich in cruciferous vegetables
are associated with lower risk of several types of cancer, scientists
are interested in the potential cancer-preventive activities of compounds
derived from glucosinolates (1). Among these
compounds is indole-3-carbinol (I3C), a compound derived from the enzymatic
hydrolysis (breakdown) of an indole glucosinolate, commonly known as glucobrassicin
(2).

Metabolism and Bioavailability

A number of commonly consumed cruciferous vegetables, including broccoli,
Brussels sprouts, and cabbage, are good sources of glucobrassicin—the
glucosinolate precursor of I3C. Myrosinase, an enzyme
that catalyzes the hydrolysis
of glucosinolates, is physically separated from glucosinolates in intact
plant cells (3). When plant cells are
damaged, as when cruciferous vegetables are chopped or chewed, the interaction
of myrosinase and glucobrassicin results in the formation of I3C (figure
1). In the acidic environment of the stomach, I3C molecules can combine
with each other to form a complex mixture of biologically active compounds,
known collectively as acid condensation products (4).
Although numerous acid condensation products of I3C have been identified,
some of the most prominent include the dimer 3,3'-diindolylmethane (DIM)
and a cyclic trimer (CT) (figure 2). The biological
activities of individual acid condensation products differ from those
of I3C and are responsible for the biological effects attributed to I3C
(5). When plant myrosinase is inactivated
(e.g., by boiling), glucosinolate hydrolysis still occurs to a lesser
degree, due to the myrosinase activity of human intestinal bacteria (6).
Thus, when cruciferous vegetables are cooked in a manner that inactivates
myrosinase, glucobrassicin hydrolysis by intestinal bacteria still results
in some I3C formation (see Food Sources). However,
acid condensation products are less likely to form in the more alkaline
environment of the intestine.

Biotransformation enzymes play major roles in the metabolism and elimination
of many biologically active compounds, including steroid
hormones, carcinogens, toxins,
and drugs. In general, phase I biotransformation enzymes, including the
cytochrome P450 (CYP)
family, catalyze reactions
that increase the reactivity of hydrophobic (fat-soluble) compounds, which prepares
them for reactions catalyzed by phase II biotransformation enzymes. Reactions
catalyzed by phase II enzymes generally increase water solubility and
promote the elimination of these compounds (7).

Acid condensation products of I3C, particularly DIM and indole[3,2-b]carbazole
(ICZ), can bind to a protein in the cytoplasm of cells called the aryl
hydrocarbon receptor (AhR) (5, 8).
Binding allows the AhR to enter the nucleus where it forms a complex with
the Ahr nuclear translocator (Arnt) protein. This Ahr/Arnt complex binds
to specific DNA sequences in genes
known as xenobiotic response elements (XRE) and enhances their transcription(9). Genes for a number of CYP enzymes
and several phase II enzymes are known to contain XREs. Thus, oral consumption
of I3C results in the formation of acid condensation products that can
increase the activity of certain phase I and phase II enzymes (8,
10, 11). Increasing the activity of biotransformation
enzymes is generally considered a beneficial effect because the elimination
of potential carcinogens or toxins is enhanced. However, there is a potential
for adverse effects because some procarcinogens require biotransformation
by phase I enzymes to become active carcinogens (12).

Alterations in Estrogen Activity and Metabolism

Endogenous estrogens, including
17beta-estradiol, exert their estrogenic effects by binding to estrogen
receptors (ERs). Within the nucleus, the estrogen-ER complex can bind
to DNA sequences in genes
known as estrogen response elements (EREs), recruit coactivator molecules, and thus enhance the transcription
of estrogen-responsive genes (13). Some
ER-mediated effects, such as those that promote cellular proliferation
in the breast and uterus, can increase the risk of developing estrogen-sensitive
cancers (14).

Effects on Estrogen Receptor Activity

When added to breast cancer cells in culture, I3C has been found to inhibit
the transcription of estrogen-responsive genes stimulated by 17beta-estradiol
(15, 16). Acid condensation products
of I3C that bind and activate AhR may also inhibit the transcription of
estrogen-responsive genes by competing for coactivators or increasing
ER degradation (9, 17).
In contrast, some studies in cell culture (18,
19) and animal models (20) have found
that acid condensation products of I3C actually enhance the transcription of estrogen-responsive
genes. Further research is needed to determine the nature of the stimulatory
and inhibitory effects of I3C and its acid condensation products on estrogen-responsive
gene transcription under conditions that are relevant to human cancer
risk (see Cancer below).

Effects on Estrogen Metabolism

The endogenous estrogen
17beta-estradiol can be irreversibly metabolized to 16alpha-hydroxyestrone
(16OHE1) or 2-hydroxyestrone (2OHE1). In contrast to 2OHE1, 16OHE1 is
highly estrogenic and has been found to stimulate the proliferation of
several estrogen-sensitive cancer cell lines (21,
22). It has been hypothesized that shifting the metabolism of 17beta-estradiol
toward 2OHE1, and away from 16OHE1, could decrease the risk of estrogen-sensitive cancers, such as breast cancer (23).
In controlled clinical trials, oral supplementation with 300-400 mg/day
of I3C has consistently increased urinary 2OHE1 levels or urinary 2OHE1:16OHE1
ratios in women (24-29). Supplementation
with 108 mg/day of DIM also increased urinary 2OHE1 levels in postmenopausal
women (30). However, the relationship
between urinary 2OHE1:16OHE1 ratios and breast cancer risk is not clear.
Although women with breast cancer had lower urinary ratios of 2OHE1:16OHE1
in several small case-control
studies(31-33), larger case-control
and prospective
cohort studies have not found significant associations between urinary
2OHE1:16OHE1 ratios and breast cancer risk (34-36).

Induction of Cell Cycle Arrest

Once a cell divides, it passes through a sequence of stages—collectively
known as the cell cycle—before it divides again. Following DNA damage,
the cell cycle can be transiently arrested at damage checkpoints, which
allows for DNA repair or activation of pathways leading to cell death (apoptosis)
if the damage is irreparable (37). Defective
cell cycle regulation may result in the propagation of mutations
that contribute to the development of cancer. The addition of I3C to prostate
and breast cancer cells in culture has been found to induce cell cycle
arrest (38, 39). However, the physiological
relevance of these cell culture studies is unclear since little or no
I3C is available to tissues after oral administration (see Metabolism
and Bioavailability) (40).

Induction of Apoptosis

Unlike normal cells, cancerous cells lose their ability to respond to
death signals that initiate apoptosis.
I3C and DIM have been found to induce apoptosis when added to cultured
prostate (38), breast (41-
43), pancreatic (44), and cervical cancer cells (45).

Inhibition of Tumor Invasion and Angiogenesis

Limited evidence in cell culture experiments suggests that I3C and DIM
can inhibit the invasion of normal tissue by cancer cells (46)
and also inhibit the development of new blood vessels (angiogenesis) required
by rapidly growing tumors (47, 48).

Disease Prevention

Cancer

Epidemiological Studies

Epidemiological studies
provide some support for the hypothesis that higher intakes of cruciferous
vegetables are associated with lower risk for some types of cancer (49).
However, cruciferous vegetables are relatively good sources of other phytonutrients
that may have protective effects against cancer, including vitamin C,
folate, selenium, carotenoids, and fiber (see Cruciferous
Vegetables). Moreover, cruciferous vegetables provide a variety of
glucosinolates, in addition to indole-3-carbinol, that may be hydrolyzed to a variety of potentially protective
isothiocyanates (e.g., sulforaphane; see Isothiocyanates) (50). Consequently,
evidence for an inverse association between cruciferous vegetable intake
and cancer risk provides relatively little information about the specific
effects of indole-3-carbinol on cancer risk.

Animal Studies

In most animal models, exposure to a chemical carcinogen
is required to cause cancer. When administered before or at the same time
as the carcinogen, oral I3C has been found to inhibit the development
of cancer in a variety of animal models and tissues, including cancers
of the mammary gland (breast) (51, 52),
uterus (53), stomach (54), colon (55, 56), lung (57), and liver (58, 59).
However, a number of studies have found that I3C actually promoted or
enhanced the development of cancer when administered chronically after
the carcinogen (post-initiation). The cancer-promoting effects of I3C
were first reported in a trout model of liver cancer (60,
61). However, I3C has also been found to promote cancer of the liver (62-64),
thyroid (64), colon (65, 66),
and uterus (67) in rats. More recently, inclusion of I3C in the maternal diet was found to protect the offspring from lymphoma and lung tumors induced by dibenzo[a,l]pyrene, a polycyclic aromatic hydrocarbon (68). Polycyclic aromatic hydrocarbons are chemical pollutants formed during incomplete combustion of organic substances, such as coal, oil, wood, and tobacco (69). Although the
long-term effects of I3C supplementation on cancer risk in humans are
not known, the contradictory results of animal studies have led several
experts to caution against the widespread use of I3C and DIM supplements
in humans until their potential risks and benefits are better understood
(62, 70,
71).

Disease Treatment

Diseases Related to Human Papilloma Virus Infection

Cervical Intraepithelial Neoplasia

Infection with certain strains of human papilloma virus (HPV) is an important
risk factor for cervical cancer (72).
Transgenic mice that express cancer-promoting HPV genes develop cervical
cancer with chronic 17beta-estradiol administration. In this model, feeding
I3C markedly reduced the number of mice that developed cervical cancer
(73). A small placebo-controlled
trial in women examined the effect of oral I3C supplementation on the
progression of precancerous cervical lesions classified as cervical intraepithelial
neoplasia (CIN) 2 or CIN 3 (74). After
12 weeks, four out of the eight women who took 200 mg/day had complete
regression of CIN and four out of the nine who took 400 mg/day had complete
regression, while none of the ten who took a placebo had complete regression.
HPV was present in seven out of the ten women in the placebo group, seven out of eight women in the 200 mg I3C group, and eight out of nine women in the 400 mg I3C group (74). Although these preliminary results are encouraging, larger controlled
clinical trials are needed to determine the efficacy of I3C supplementation
for preventing the progression of precancerous lesions of the cervix (75).

Vulvar Intraepithelial Neoplasia

HPV infection can also lead to vulvar intraepithelial neoplasia (VIN) (76). A small randomized trial in 12 women with VIN found that supplementation with 200 mg/day or 400 mg/day of I3C for six months improved overall symptoms and decreased lesion size and degree of aggressive histopathology (77). While the results of this preliminary trial are promising, more clinical trials are needed to determine whether I3C might be an effective treatment for VIN.

Recurrent Respiratory Papillomatosis

Recurrent respiratory papillomatosis (RRP) is a rare disease of children
and adults, which are characterized by generally benign growths (papillomas) in
the respiratory tract caused by HPV infection (78).
These papillomas occur most commonly on or around the vocal cords in the
larynx (voice box), but they may also affect the trachea, bronchi, and
lungs. The most common treatment for RRP is surgical removal of the papillomas.
Since papillomas often recur, adjunct
treatments may be used to help prevent or reduce recurrences (79).
In immune-compromised mice transplanted with HPV-infected laryngeal tissue,
only 25% of the mice fed I3C developed laryngeal papillomas compared
to 100% of the control mice (80). In
a small observational
study of RRP patients, increased ratios of urinary 2OHE1:16OHE1 ratios
resulting from increased cruciferous vegetable consumption were associated
with less severe RRP (81). Most recently,
an uncontrolled pilot study examined the effect of I3C supplementation
(400 mg/day for adults and 10 mg/kg daily for children) on papilloma recurrence
in RRP patients (82). Over a 5-year
follow-up period, 11 of the original 49 patients experienced no recurrence,
ten experienced a reduction in the rate of recurrence, 12 experienced no
improvement, and 12 were lost to follow-up (83).
Although the low toxicity of I3C makes it an attractive adjunct therapy
for RRP, controlled clinical trials are needed to determine whether I3C
is effective in preventing or reducing the recurrence of respiratory papillomas.

Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune
disorder characterized by chronic inflammation that may result in
damage to the joints, skin, kidneys, heart, lungs, blood vessels, or brain
(84). Estrogen is thought to play a role
in the pathology of SLE because the disorder is much more common in women
than men, and its onset is most common during the reproductive years when
endogenous estrogen levels are highest (85).
The potential for I3C supplementation to shift endogenous estrogen metabolism
toward the less estrogenic metabolite 2OHE1, and away from the highly estrogenic
metabolite 16OHE1 (see Estrogen Metabolism), led
to interest in its use in SLE (24). In an animal model of SLE, I3C feeding
decreased the severity of renal (kidney) disease and prolonged survival
(86). A small uncontrolled trial of I3C
supplementation (375 mg/day) in female SLE patients found that I3C
increased urinary 2OHE1:16OHE1 ratios, but the trial found no significant change
in SLE symptoms after three months (86).
Controlled clinical trials are needed to determine whether I3C supplementation
will have beneficial effects in SLE patients.

Sources

Food Sources

Glucobrassicin, the glucosinolate precursor of I3C, is found in a number
of cruciferous vegetables, including broccoli, Brussels sprouts, cabbage,
cauliflower, collard greens, kale, kohlrabi, mustard greens, radish, rutabaga,
and turnip (87, 88). Although glucosinolates
are present in relatively high concentrations in cruciferous vegetables,
glucobrassicin makes up only about 8-12% of the total glucosinolates (89).
Total glucosinolate contents of selected cruciferous vegetables are presented
in the table below (90). The amount of
indole-3-carbinol formed from glucobrassicin in foods is variable and
depends, in part, on the processing and preparation of foods.

Effects of Cooking

Glucosinolates are water-soluble compounds that may be leached into cooking
water. Boiling cruciferous vegetables from 9-15 minutes resulted in 18-59%
decreases in the total glucosinolate content of cruciferous vegetables
(90). Cooking methods that use less water,
such as steaming or microwaving, may reduce glucosinolate losses. Some
cooking practices, including boiling (91),
steaming (92), and microwaving at high
power (850-900 watts) (93, 94), may inactivate myrosinase, the enzyme that catalyzes
glucosinolate hydrolysis. Even
in the absence of plant myrosinase activity, the myrosinase activity of
human intestinal bacteria results in some glucosinolate hydrolysis (6).
However, studies in humans have found that inactivation of myrosinase
in cruciferous vegetables substantially decreases the bioavailability
of glucosinolate hydrolysis products known as isothiocyanates (91-93).
Since the formation of I3C also depends on glucosinolate hydrolysis, it
is very likely that the bioavailability of I3C and its acid condensation
products would also be decreased by myrosinase inactivation.

I3C is available without a prescription as a dietary supplement. I3C
supplementation increased urinary 2OHE1 levels in adults at doses of 300-400
mg/day (28). I3C doses of 200 mg/day or 400
mg/day improved the regression of cervical intraepithelial neoplasia (CIN)
in a preliminary clinical trial (74). I3C in doses up to 400
mg/day has been used to treat recurrent respiratory papillomatosis (82,
83). These supplemental levels are well above levels dietary levels, which commonly range from 20-120 mg daily (95).

3,3'-Diindolylmethane (DIM)

DIM is available without a prescription as a dietary supplement. In a
small clinical trial, DIM supplementation at a dose of 108 mg/day for
30 days increased urinary 2OHE1 excretion in postmenopausal women with
a history of breast cancer (30).

Safety

Adverse Effects

Slight increases in the serum
concentrations of a liver enzyme (alanine aminotransferase; ALT) were
observed in two women who took unspecified doses of I3C supplements for
four weeks (28). One person reported
a skin rash while taking 375 mg/day of I3C (24).
High doses of I3C (800 mg/day) were associated with symptoms of disequilibrium
and tremor, which resolved when the dose was decreased (82).
I3C supplementation enhanced the development of cancer in some animal
models when given after the carcinogen (62, 64, 65, 67) (see Cancer).
The effects of I3C or DIM supplementation on cancer risk in humans are
not known.

Pregnancy and Lactation

The safety of I3C or DIM supplements during pregnancy or lactation has
not been established.

Drug Interactions

No drug interactions in humans have been reported. However, preliminary
evidence that I3C and DIM can increase the activity of CYP1A2 (96,
97) suggests the potential for I3C or DIM supplementation to decrease
serum concentrations of medications metabolized by CYP1A2. Both I3C and
DIM modestly increase the activity of CYP3A4 in rats when administered
chronically (98). This observation raises
the potential for adverse drug interactions in humans since CYP3A4
is involved in the metabolism of approximately 50% of therapeutic drugs.

The Linus Pauling Institute Micronutrient Information Center provides scientific information on the health aspects of dietary factors and supplements, foods, and beverages for the general public. The information is made available with the understanding that the author and publisher are not providing medical, psychological, or nutritional counseling services on this site. The information should not be used in place of a consultation with a competent health care or nutrition professional.

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